1. Field of the Invention
This invention relates to a television receiver and a cold-cathode tube dimmer for controlling the brightness of a cold-cathode tube by controlling the ratio of the interval during which the lighting/driving of the cold-cathode tube is done to the interval during which the lighting/driving of the cold-cathode tube is stopped.
2. Description of the Related Art
In the case where the brightness of a cold-cathode tube is controlled in such a manner as to change the voltage of a lighting/driving signal for lighting the cold-cathode tube, when the voltage of the lighting/driving signal is lowered, a phenomenon occurs that the end of the side on which a minus voltage is applied becomes dark. This makes a dimmable range narrow. In order to overcome such an inconvenience, various prior arts have been proposed in which the luminance of the cold-cathode tube is controlled by changing the duty ratio of the lighting/driving signal.
One of these prior arts (first prior art) will be explained below. In this prior art, as shown in FIG. 4, the current flowing through a fluorescent lamp 91 is converted into a voltage using a resistor R91. The voltage across the resistor R91 is rectified and smoothed by a diode D91 and capacitor C91 to provide a signal 92 which represents the average value of the current as a voltage. By controlling the on/off ratio of the power source for an inverter for driving the fluorescent lamp 91 on the basis of the average value of the current represented by the signal 92, the brightness of the fluorescent lamp 91 is controlled to a prescribed brightness. Further, on the basis of the output obtained through division of the power source voltage for the inverter circuit using a potentiometer circuit, the power source voltage is detected. On the detected result of the power source voltage, the on/off ratio of the power source of the inverter circuit is controlled simultaneously (for example, see JP-A-11-122937).
The following prior art (second prior art) has also been proposed. In this prior art, a pulse of which the duty ratio varies according to the resistance of a variable resistor is supplied to the base of the transistor which constitutes the inverter circuit for lighting/driving the fluorescent lamp. Therefore, when the resistance of the variable resistor is varied, the duty ratio when the fluorescent lamp is driven by the inverter circuit varies. Thus, the brightness of the fluorescent lamp tube can be varied by varying the resistance of the variable resistor (see Japanese Patent No. 3038717).
The following prior art (third prior art) has been also proposed. Referring to FIG. 4 (it is assumed that reference numeral 91 designates a cold-cathode tube), this prior art will be explained. The voltage across the resistor R91 connected in series with the cold-cathode tube 91 is rectified by the diode D91, and smoothed by the capacitor C91. Further, a resistor R92 is connected in parallel to the capacitor C91. On the basis of the output 92 thus rectified and smoothed, a PWM signal is created. Using a DC signal obtained from the PWM signal thus created, the voltage of an operating power source to be supplied to the inverter circuit is controlled (for example, see JP-A-6-302387).
The following prior art (fourth prior art) has been further proposed. In this prior art, the voltage across the resistor connected in series with a discharge tube is amplified using an amplifier and the amplified voltage is thereafter added to an adder unit. The voltage generated across the resistor and amplified by the amplifier and a signal for dimming are added in the adder unit. Thus, the brightness of the discharge tube can be controlled to that corresponding to the signal for dimming (for example, see JP-A-2002-124395 (paragraph [0017], FIG. 1)).
However, the first prior art presented the following problem. Specifically, as described in paragraph [0015], the signal 92 is a signal obtained “by extracting the average voltage of a rectified voltage by the diode D91 and capacitor C91”. This configuration provides a low-pass filter having a frequency characteristic of removing most of the frequency component of the signal for controlling the on/off duty ratio of the inverter circuit. Therefore, assuming that the current flowing through the fluorescent lamp 91 during driving remains constant, a level difference corresponding to the duty ratio does not occur between the level change (indicated by 92a) when the duty ratio of the lighting/driving signal is increased as indicated by 93a in FIG. 5 and the level change (indicated by 92b) when the duty ratio of the lighting/driving signal is increased (V92a and V92b are equal to each other). On the other hand, if the current flowing through the fluorescent lamp 91 during driving becomes a low level, the level of the signal 92 lowers as indicated by 92c (V92c becomes lower than V92a). Namely, although the signal 92 can accurately represent the variation in the current flowing through the fluorescent lamp, it cannot accurately represent the variation in the duty ratio with a high accuracy. In other words, the signal 92 is a signal with a very low accuracy for the variation in the duty ratio. As a result, in the case where the duty ratio of the lighting/driving signal is controlled in a closed loop using the signal 92, the precision of control is deteriorated, thereby providing a larger variation in the luminance. Thus, in the case where the this prior art is applied to the backlight for a television receiver equipped with a liquid crystal display, the signal is susceptible to the influence of changes in the power source voltage and temperature, thereby providing a variation in the brightness on the screen.
The above problem is brought by the third prior art. However, the third prior art does not adopt the method of controlling the brightness of the cold-cathode tube by changing the duty ratio of the lighting/driving signal. Therefore, the above defect is not problematic. However, since the configuration as a premise is different, it is difficult to apply the third prior art to the case where the brightness of the cold-cathode tube should be stabilized accurately in an apparatus for controlling the brightness by changing the duty ratio of the lighting/driving signal for driving the cold-cathode tube.
Further, although the second prior art controls the brightness of the fluorescent tube by changing the duty ratio of the lighting/driving signal, the method of control is implemented in an opened loop. Therefore, like the third prior art, it is difficult to apply this second prior to the closed loop control in which the current flowing through the cold-cathode tube is detected and the duty ratio of the lighting/driving signal for driving the cold-cathode tube is changed according to the result of detection.
Further, the fourth prior art does not describe in detail how the output from an error voltage detecting circuit is processed in the adder unit. Therefore, it is difficult to apply this fourth prior art to the case where the brightness of the cold-cathode tube should be stabilized accurately in a configuration for controlling the brightness in a configuration in which the current flowing through the cold-cathode tube is detected and the duty ratio of the lighting/driving signal for driving the cold-cathode tube is changed according to the result of detection.